1. Field of the Invention
The present invention relates to an exhaust aftertreatment system.
2. Description of the Related Art
Exhaust gas from a diesel engine is generally purified by a catalyst arranged in an exhaust pipe through which the exhaust gas flows. Known as this kind of catalyst is an NOx-absorption reduction catalyst which has a property of oxidizing NOx in exhaust gas to temporarily absorb the same in the form of nitrate when an air/fuel ratio of the exhaust gas is lean, and conducting decomposition into NOx for reduction and purification thereof with the assistance of unburned HC and CO when the oxygen concentration in the exhaust gas is lowered.
Known as this kind of NOx-absorption reduction catalyst having the above-mentioned property is, for example, a catalyst made from alumina and carrying platinum and barium or a catalyst made from alumina and carrying iridium, platinum and barium.
Particulates or particulate matter in the exhaust gas is mainly constituted by carbonic soot and a soluble organic fraction of high-boiling hydrocarbons and contains a trace of sulfate or misty sulfuric acid fraction. In order to reduce a discharged amount of particulates from the engine, a particulate filter is conventionally arranged in the exhaust pipe through which the exhaust gas flows.
FIG. 1 shows a conventional exhaust aftertreatment system in which reference numeral 1 generally denotes a diesel engine as an internal combustion engine. The engine 1 shown has a turbocharger 2 comprising a compressor 2a and a turbine 2b. Suction air 4 is guided via an air cleaner 3 and a suction pipe 5 into the compressor 2a of the turbocharger 2 for pressurization; the pressurized air 4 is cooled in an intercooler 6 and then is distributed via an intake manifold (not shown) into respective cylinders of the engine 1.
Concurrently, fuel or diesel oil is injected by a fuel injection device 13 into the respective cylinders of the engine 1 for combustion; exhaust gas 8 discharged from the cylinders of the engine 1 is fed via an exhaust manifold 7 into the turbine 2b of the turbocharger 2. The exhaust gas 8 having driven the turbine 2b is discharged out of a vehicle via an exhaust pipe 9.
Arranged in the exhaust pipe 9 is a casing 10 which accommodates an upstream NOx-absorption reduction catalyst 11 and a downstream particulate filter 12 integrally carrying an oxidation catalyst. Also arranged in the exhaust pipe 9 and upstream of the casing 10 is an exhaust brake 14 which is adjustable in opening degree so as to throttle the flow path of the exhaust pipe 9 to a desired opening degree.
A specific structure of the particulate filter 12 is shown in FIG. 2. The particulate filter 12 is of a porous honeycomb structure made of ceramics such as cordierite and having lattice-like compartmentalized passages 12a. Alternate ones of the passages 12a have plugged inlets and the remaining passages 12a with unplugged open inlets are plugged at their outlets. Thus, only the exhaust gas 8 passing through thin porous compartment walls 12b which define the passages 12a is discharged downstream while the particulates are captured and accumulated on inner surfaces of the walls 12b. The oxidation catalyst is coated to the whole of the filter 12 to an extent not to bring about clogging.
More specifically, according to the exhaust aftertreatment system shown in FIG. 1, NOx in the exhaust gas 8 is absorbed in the NOx-absorption reduction catalyst 11 in the form of nitrate so as to reduce NOx, and the particulates in the exhaust gas 8 are captured by the particulate filter 12, whereby purified exhaust gas 8 is discharged out of the vehicle.
The above-mentioned exhaust aftertreatment system with the particulate filter is disclosed, for example, in JP 2001-317332 A.
However, there exists SO2 in the exhaust gas 8 from the engine 1 which derives from sulfur in the fuel. Such SO2 is oxidized on the NOx-absorption reduction catalyst 11 just like NOx, disadvantageously resulting in sulfate. Sulfate is more stable than nitrate so that the NOx-absorption reduction catalyst 11 is poisoned and deteriorated by sulfate, leading to a problem that absorption of NOx becomes impossible and NOx purification ratio is lowered. There has been no specific proposals which have been developed to a practical level on the problem.
A technique itself for overcoming the poisoning through sulfate is known; rich combustion with an air/fuel ratio lower than theoretical air/fuel ratio and maintaining the NOx-absorption reduction catalyst 11 in a high-temperature atmosphere of approximately 650° C. or more will release SO2 from the reduction catalyst 11, thereby regenerating the reduction catalyst 11. This is difficult to carry out by controlling an operation of the diesel engine 1. Moreover, if the temperature of the exhaust gas 8 becomes approximately 700° C. or more, catalyst material of the NOx-absorption reduction catalyst 11 chemically combined with sulfur will be crystallized so that detachment of sulfur becomes impossible and deterioration gradually progresses with no recovery of catalytic performance of the reduction catalyst 11, disadvantageously leading to a short service life of the reduction catalyst 11.
The invention was made in view of the above and has its object to provide an exhaust aftertreatment system which can prevent the NOx-absorption reduction catalyst from being poisoned and deteriorated through sulfate without difficultly controlling an operation of a diesel engine, can prevent the ratio of NOx purification through the NOx-absorption reduction catalyst from being lowered and can prolong the service life thereof.